Intelligence reports on the Syrian chemical attack far from prove that President Bashar al-Assad was responsible, US officials said last night.

One admitted the evidence was ‘not a slam dunk’ - a pointed reference to language used by the CIA to describe intelligence on weapons of mass destruction ahead of the Iraq war.

Despite those doubts, United States is expected to go it alone to launch a limited airstrike on Syria.

Military intervention: A US Air Force plane lands at Incirlik Air Base in Turkey yesterday. United Nations Secretary-General Ban Ki-moon has said the inspection team in Syria is expected to complete its work by today

President Barack Obama is prepared to go ahead despite last night’s rejection of military action by Britain, according to senior US administration officials.

Last night the White House briefed national security staff and senior lawmakers to lay the groundwork for missile strikes, which could come as soon as tomorrow.

On Wednesday, President Obama declared that the Syrian regime was responsible for the chemical attack.

Share this article

Yesterday he ordered a classified report which makes America’s case for intervention to be made public.

But Washington officials insisted that the report was ‘not a slam dunk’. The basketball term was a deliberate reference to claims made by former CIA director George Tenet in 2002.

He said US intelligence showing that Saddam Hussein had weapons of mass destruction in Iraq was a ‘slam dunk.’ That intelligence turned out to be wrong.

Plan: President Barack Obama is prepared to go ahead with a limited airstrike on Syria despite last night¿s rejection of military action by Britain, according to senior US administration officials

The declassified report does not
contain a smoking gun directly linking Assad to the massacre that killed
at least 350 people last week, officials said.

American
satellites have captured images of Syrian troops moving trucks into
weapons storage areas and removing materials, but analysts have not been
able to track what was moved or, in some cases, where it was relocated.

This means a possible series of US cruise missile strikes aimed at crippling Assad’s military infrastructure could hit newly hidden supplies of chemical weapons, accidentally triggering another attack.

The US also intercepted a phone call in which Syrian military officials discussed the strike, but the staff were only low-level, the officials said.

It did not contain any direct evidence tying the attack back to Assad or even a senior Syrian commander – the link the White House is looking for.

It wants to rule out the possibility that a rogue element of the military decided to use chemical weapons without authorisation.

So while Secretary of State John Kerry said on Monday that links between the attack and the Assad government were ‘undeniable,’ US intelligence officials are not certain that it was carried out on Assad’s orders.

Claim: Secretary of State John Kerry said on Monday that links between the attack and the Assad government were 'undeniable'

They are not even completely sure it was carried out by government forces.

Despite these reservations, Mr Obama was said to be confident that there was sufficient evidence to act once UN inspectors have left Syria tomorrow.

The strike is likely to be a cruise missile bombardment from US destroyers in the eastern Mediterranean, probably only lasting for a couple of days.

A fifth US warship was expected to arrive in the region today.

A US State Department official said President Obama would not wait for Britain, explaining: ‘We can’t wait. We need to act according to our own national interests. We need to act. Why do we need to wait for a UN report to tell us what we already know?’

Asked why the US had to go it alone in Syria while Europe hesitated, congressman Eliot Engel, a member of the House Foreign Affairs Committee, said: ‘It’s difficult to answer that question.’

He didn’t openly criticise Britain, but added: ‘As the US we stand for something. We stand for democracy. We stand for human rights. In the US we have principles.

'These are difficult decisions. We don’t take them lightly. I think the world has to show its revulsion and this is a way of doing it. The worst choice we can do is to do nothing.’

Russia is sending an anti- submarine battleship and a missile cruiser to the eastern Mediterranean in a move described as ‘gunboat diplomacy’ over Syria. Experts say the bolstering of Moscow’s naval presence could potentially give the Assad regime early warning of Western submarine cruise missile launches, or jam radars and navigation systems.